The unique features and advantages of the vibrating bed pyrolysis system are best put in perspective by a review of representative systems of the many that have been devised and operated during the past 200 years in efforts to produce liquid and gaseous fuels from coal. In the first of these, byproduct gas from beehive coke ovens came into commercial use about 1800, and soon became so widely used for street lighting that the demand exceeded the supply. The gas yield per ton of coal was greatly increased by periodically shutting off the air and injecting water or steam against the red hot coke to generate H.sub.2 O and CO via the water gas reaction, thus consuming all of the coke to produce illuminating gas. This basic method of generating gaseous fuel was widely used until pipelines made less expensive natural gas available in the 1920 to 1960 period. Coke production continued in beehive ovens with recovery of some hydrocarbon liquids as byproducts. These byproducts included "coal oil" which gave a better fuel for oil lamps than whale oil, and was much less expensive.
In 1881 a retort type of coke oven was developed in Germany to minimize the amount of oxygen to which the coal was exposed in the pyrolysis process and thus produce both a better quality of coke and more high value hydrocarbon byproducts. In this approach the coal is placed in a set of slab-shaped chambers roughly a foot thick, 10 ft high, and 20 to 40 ft long. These chambers are placed side by side to form a huge oven in which the coal is heated to about 1800.degree. F. by hot combustion gases circulated through passages in the walls between adjacent chambers. The time for heating the coal charge is long because of the long path for heat flow, and the rate of gas evolution is low because of the compactness of the charge, hence the residence time required per charge is 10 to 20 h. From 1 to 2% of the weight of the coal is given off as condensable hydrocarbons, and about 3% as combustible gases. About a third of the gas is employed to produce the hot combustion gases used to heat the coal. This coking process is still the prime source of coke at the end of the 20th century.
The most successful of the many efforts to develop a superior system for gasifying coal via the water gas reaction is the Lurgi system brought out in Germany in the 1920's. To increase the throughput rate, and thus reduce the capital costs, coal is fed into the top of a large cylindrical vessel, air and steam flow up through a grate at the bottom, and the mass of solids above the grate is stirred slowly to keep the charge mixed and to increase the effective surface area. Ash is drained from the bottom and the gas produced flows out the top of the vessel.
A vital element in Hitler's preparations for World War II included an intensive program to synthesize liquid hydrocarbons for motor fuel from the "wasser gas" produced with Lurgi gasifiers. The program was sufficiently successful that it produced as much as 70,000 barrels per day of motor fuel during WWII, a vital input to the Nazi war effort, but only about 3% of the U.S. petroleum production at that time. Thus, except for the Panzer divisions, much of the German army was horse-drawn.
After WWII, military planners in the U.S. became concerned with the impending depletion of U.S. petroleum reserves, and instigated a program to synthesize motor fuel from coal. This led to a major U.S. national program that took the German work as the point of departure, with improvements derived from petroleum refinery experience including operation at higher temperatures and pressures to improve the yield and the process efficiency. Fluidized beds of coal and relatively inert particles were employed to give a much higher rate of mixing and greater effective surface area than the moving bed in a Lurgi, thus increasing reaction and throughput rates and reducing the capital costs. These U.S. efforts slowly increased in scale until the "Energy Crisis" in the 1970's led to the construction of a series of demonstration plants. Serious troubles with corrosion, coking in retorts, and clogging of lines by tarry deposits together with rapidly escalating estimates of costs led to the cancellation of one after another of these expensive large-scale projects until virtually the entire program was terminated. By 1990 the only large scale plant in the world producing liquid motor fuel from coal was the SASOL plant in South Africa which was heavily subsidized by the government to provide a source of motor fuel in the event of a petroleum embargo. This plant makes use of Lurgi gasifiers and WWII German technology.
A need exists for an improved system for producing fluid fuels and feed stocks from solid hydrocarbon products.